1) My tank is 5.5 gallons
2)freshwater
3)tank has been set up for 2 weeks
5)I completely cycled the aquarium before introducing the betta fish
4) one Betta splendens
5)only artificial plants
6)temperature is a constant 80 F
7)filter is a Hydor sponge filter and gravel substrate
8. I don't use a CO2 unit
9)tank doesn't really receive natural sunlight, fluorescent 8 watt lighting for 12 hours
10)last water change: today; quantity: 3 quarts
11)water changes once per week (except I was gone for about 10 days and didn't have my friend do a water change since he is not familiar with aquarium maintenance.
12)Feeding: once per day Foods: freeze dried blood worms and New Life Spectrum Marine fish formula pellets, but changing pellets to HBH Betta Bites.
13)The first 6 days I had the Betta fish, it would vigorously eat the blood worms. I tried feeding New Life Spectrum Marine fish formula pellets, but it spit them out, so I just fed him just enough freeze dried blood worms twice per day. After that he gradually lost his ability to see and now cannot see where he is swimming and thus also not find his food. He is also breathing with his gills a tad faster. When I first bought him he actively swam around the aquarium exploring and now he is lethargic and often sits on the bottom. When I look straight into either of his eyes they look cloudy. His irises are as dark as his pupils. I don't know what he has. Possibly a bacterial infection. Also, his abdomen appears more rounded than it should be. I have had little experience with sick fish in at least 5 years of fish keeping.
14)pH: 7.4 according to a Wardley Junior pH kit capable of measuring from 6.2-7.4; Ammonia: 0 ppm, Nitrite: 0 ppm, Nitrate: somewhere between 40 and 80 ppm (NH3, NO2(-), and NO3(-) were measured with API liquid test kits)
15)The last time I bought a fish was in the spring of 2006. My betta fish was not very active in the small cup he was in when in the pet store.

Thank you for reading all this information. I'm just following the request shown in the sticky thread titled MUST READ: Health Issues for Bettas. Please help me figure this out and I will try to answer any of your questions.